1. Field of the Invention
The present invention relates to a display device used for a portable information device such as a mobile phone or a mobile computer, and an illuminating device used for the display device.
2. Description of the Related Art
Up to now, there has been used a liquid crystal display device which is small in size and thin in thickness and has low power consumption as a color image display device for most of portable information devices such as mobile phones or mobile computers.
The liquid crystal display device includes an illuminating device. An edge-light type has been widely known as the type of the illuminating device. In the edge-light type illuminating device, light from a light source is made incident on a side surface of a light guide plate and then emitted from an upper surface (hereinafter, referred to as light emission surface) of the light guide plate. The light source to be used is a point light source such as a cold-cathode tube or a light emitting diode (LED). A large number of groove portions or dot patterns are formed in a surface (hereinafter, referred to as opposed surface) opposite to the light emission surface of the light guide plate. Diffusion patterns having a light diffusion effect are formed in the light emission surface in many cases. A prism is formed in a light incident surface of the light guide plate (that is, surface which is opposed to light source and on which light from light source is made incident) and has a function of diffusing light from the point light source to cause surface emission. A material of the light guide plate which is to be used is a transparent resin such as polycarbonate (PC) or acrylic (PMMA), which has a higher refractive index than air. In general, a diffusion sheet or a prism sheet is provided on the light emission surface side of the light guide plate and a reflective sheet is provided under the light guide plate.
In order to specialize in the point light source such as the LED and maintain predetermined luminance even in a case where the prism sheet is not provided, the development of a light guide plate using a micro prism array has been advanced (see, for example, JP 2007-280952 A, FIG. 6).
A conventional illuminating device has limitations on thickness, luminance efficiency, and cost because of the use of the prism sheet. In the case of the micro prism array type illuminating device disclosed in JP 2007-280952 A, square pyramid diffusion patterns are fundamentally aligned in the same direction, and hence it is difficult to sufficiently use light from the point light source. The illuminating device has substantially the same luminance efficiency level as in the case where the prism sheet is used, and thus has a very low use efficiency of light from the light source.
The conventional micro prism array type illuminating device has only a single light source, and hence it is difficult to increase the number of light sources to two or more.
An illuminating device with a structure in which the prism sheet is not used has a problem that a moire fringe (interference) is more likely to occur in a case where a liquid crystal panel is mounted.